Amber is the fossilized resin of prehistoric trees.

When it was excreted in ancient forests, the molecules became polymerized and cross-linked over millions of years, creating a solid fossil. The oldest amber has been dated to 320 million years old.

This reddish amber is from the Indonesian island of Sumatra and is approximately 10–30 million years old. There are occasionally fossilized insects trapped inside Sumatran amber, but its dark color can make them difficult to spot. Try holding your specimen in front of a light to check for yourself.

Amber is very soft, only about a 2 on the Mohs scale. Sumatran amber has the added property of fluorescing bright blue under UV light.